Titania or titanium dioxide (TiO2) is generally present in the market place in either of two predominant polymorphs, anatase or rutile, and has an average particle size of 150 nm to 250 nm. Because of its high refractive index, negligible color and inertness, it is useful as an opacifier in paints, paper, plastics, ceramic, inks, etc. Titanium dioxide having a smaller average particle size, for example in the 1 nm to 150 nm average particle size range is referred to as nano titania. Nano titania finds uses in cosmetics, personal care products, plastics, surface coatings, self-cleaning surfaces and photovoltaic applications due to its: i) translucency combined with photoprotective properties; ii) photocatalytic properties; and iii) conductive properties combined with high surface area.
Issues surrounding nano titanium dioxide products supplied in powder form include: i) the difficulties encountered in dispersing the products to the required size; and ii) dust/handling concerns due to the fineness of the nano titania powder. A nano titanium dioxide product supplied in the form of a stable, concentrated sol would address both of these critical issues.
Stable aqueous nano titanium dioxide sols have been reported throughout the literature. However, their production has been limited to either very low (<pH 2) or very high (>pH 10) pH regions or to low concentrations (<300 gpl).
In U.S. Pat. No. 2,448,683, a process is described in which hydrous titanium dioxide is first neutralized then peptized with HCl to form a colloidal titanium dioxide sol. The colloidal titanium dioxide sol is then neutralized, dried and calcined at a temperature ranging from about 500°-600° C. before the calcined product is re-dispersed.
In U.S. Pat. Publ. No. 2006/0110319, rutile nano-sized titanium dioxide sols are prepared via the hydrolysis of titanium tetraisopropoxide in an aqueous solution containing hydrogen peroxide and subsequently hydrothermally treated at a temperature of 50°-120° C.
U.S. Pat. Publ. No. 2009/0062111 describes a process for forming a nano-sized titanium dioxide sol including the precipitation of hydrous titanium dioxide from a titanium iospropoxide solution and peptization of the hydrous titanium hydroxide with an α-hydroxy carboxylic acid at a temperature of 70°-150° C. for an extended period of time.
U.S. Pat. Publ. No. 2009/0061230 also describes a process for forming a stable nano-sized titanium dioxide sol through the reaction of a halide-containing titanium compound with water in the presence of a polyol.
In U.S. Pat. No. 5,840,111, a nano-sized titanium dioxide sol is prepared by the addition of a solution of sulphuric acid and titanyl sulphate to an alkaline reacting medium to form titanium dioxide nanoparticles, flocculating the nanoparticles through the addition of a monobasic acid and then isolating the flocculate by filtration.
As such, alternative means for producing highly stable nano-sized titanium dioxide sols are still highly desirable, and in particular, means for producing highly stable concentrated nano-sized titanium dioxide sols.